1. Field of the Invention
The present invention relates to a reflective display device and a method for fabricating such a display device.
2. Description of the Related Art
A reflective liquid crystal display device for conducting a display operation by utilizing ambient light as its light source has been known in the art. Unlike a transmissive liquid crystal display device, the reflective liquid crystal display device needs no backlight, thus saving the power for light source and allowing the user to carry a smaller battery. Also, the space to be left for the backlight in a transmissive device or the weight of the device itself can be saved. For that reason, the reflective liquid crystal display device is not only effectively applicable to various types of mobile electronic units that should be as lightweight and as thin as possible but also allows the use of a battery of an increased size as compared to a unit including the transmissive device. This is because the space to be left for a backlight in the transmissive device can be used for other purposes in the reflective display device. Thus, the reflective liquid crystal display device is expected to increase the longest operating time of those units by leaps and bounds.
Also, an image presented by a reflective liquid crystal display device has a better contrast than an image presented by a display device of any other type even when the display device is used outdoors in the sun. For example, when a CRT, a self-light-emitting display device, is used outdoors in the sun, the contrast ratio of an image presented thereon decreases significantly. Likewise, even a transmissive liquid crystal display device, subjected to low reflection treatment, also presents an image at a significantly decreased contrast ratio when the device is operated in an environment in which the ambient light is much intenser than the display light (e.g., in direct sunshine). On the other hand, a reflective liquid crystal display device increases the intensity of the display light proportionally to the quantity of the ambient light, thus avoiding the significant decrease in contrast ratio even where the ambient light is intense. For that reason, a reflective liquid crystal display device can be used particularly effectively in mobile electronic units that are often used outdoors, e.g., personal digital assistants, digital cameras and camcorders.
Even though the reflective liquid crystal display devices have these advantageous features that are very useful in various applications, the reflective devices currently available are not fully satisfactory yet in terms of their contrast ratio in dark places, definition, and full-color and moving picture presentation capabilities, for example. Thus, the development of more practically useful, reflective color liquid crystal display devices is awaited.
To enhance the display performance of reflective color liquid crystal display devices, a reflective liquid crystal display device using a retroreflector (which will be referred to herein as a “retroreflective liquid crystal display device”) was proposed in Japanese Patent Application Laid-Open Publication No. 2003-195788 and Japanese Patent Application Laid-Open Publication No. 2003-255373, which were filed by the applicant of the present application. A “retroreflector” is an element for reflecting an incoming light ray by a plurality of reflective surfaces back toward its source, no matter where the light ray has come from. And the retroreflector has a structure in which very small unit structures are arranged two-dimensionally, e.g., an array of microspheres, an array of microlenses, and an array of corner cubes.
Hereinafter, the operation principle of a retroreflective liquid crystal display device will be described with reference to FIGS. 1A and 1B, which schematically illustrate the black and white display modes of the display device, respectively.
As shown in FIG. 1A, if a liquid crystal layer 1 is controlled to exhibit a transmitting state, an incoming light ray 3, which has been emitted from a light source 5 outside of the display device, is transmitted through the liquid crystal layer 1 and then reflected back by a retroreflector 2 toward its light source 5 as pointed by the arrow 4b. Thus, the light ray 3 that has been emitted from the light source 5 does not reach the eyes of a viewer 6. In such a state, the image reaching the eyes of the viewer 6 from this display device is the image of his or her own eyes. In this manner, the “black” display mode is realized.
On the other hand, if the liquid crystal layer 1 is controlled to exhibit a scattering state, the incoming light ray 3 that has been emitted from the light source 5 is scattered by the liquid crystal layer 1 as shown in FIG. 1B. Specifically, if the liquid crystal layer 1 is a forward scattering liquid crystal layer, the scattered light is reflected by the retroreflector 2 toward the viewer 6 through the liquid crystal layer 1 in the scattering state (as pointed by the arrows 4w). In this case, since the retroreflectivity of the retroreflector 2 is disturbed by the scattering caused by the liquid crystal layer 1, the incoming light ray 3 does not return to its light source, thus realizing a “white” display mode.
By conducting a display operation based on this operation principle, a monochrome display is realized without using any polarizer. Consequently, a high-brightness reflective liquid crystal display device, of which the optical efficiency is not decreased by the use of polarizers, is realized.
FIG. 2 is a cross-sectional view illustrating a configuration for a conventional retroreflective liquid crystal display device that utilizes this operation principle.
The display device 100 includes a front substrate 10, on which a plurality of color filters 19, a transparent counter electrode 18 and a liquid crystal alignment film (not shown) are stacked, a rear substrate 12 arranged so as to face the front substrate 10, and a liquid crystal layer 1 interposed between these substrates 10 and 12. The rear substrate 12 includes an interlevel dielectric layer 20, a reflective layer 2 and an alignment film (not shown), which are stacked in this order on a TFT substrate including a plurality of switching elements such as thin-film transistors (TFTs). The interlevel dielectric layer 20 has a surface shape that achieves retroreflectivity. In this display device 100, the reflective layer 2 has been formed on the interlevel dielectric layer 20 and has unevenness corresponding to the surface shape of the interlevel dielectric layer 20, thus functioning as a retroreflector. Also, the reflective layer 2 is made up of a plurality of pixel electrodes 8, which are arranged separately from each other for respective pixels as units of an image presented. Each of those pixel electrodes 8 in the reflective layer 2 is connected to the drain electrode 15 of its associated switching element 14 by way of a contact portion 16 that has been formed through the interlevel dielectric layer 20. The liquid crystal layer 1 may be a scattering liquid crystal layer that can switch between the light transmitting state and the light (forward) scattering state by changing the voltages applied between the counter electrode 18 and each of those pixel electrodes.
To further increase the contrast ratio on the screen of the conventional retroreflective liquid crystal display device shown in FIG. 2, it is important to maximize the retroreflectivity of the reflective layer 2 and thereby minimize the quantity of unwanted reflected light reaching the viewer's eyes in the black display mode. Thus, a display device including a corner cube array, which is one of retroreflectors realizing the highest retroreflectivities, was proposed in Japanese Patent Application Laid-Open Publication No. 2003-195788, for example. More specifically, the display device uses a corner cube array as the interlevel dielectric layer 20, on which a reflective layer 2 with the corner cube array shape has been deposited. As used herein, the “corner cube array” is a two-dimensional array of corner cubes, each defined by three planes that are opposed substantially perpendicularly to each other. A light ray that has entered a corner cube is ideally reflected back toward its source by the three planes that form the corner cube.
FIGS. 3A and 3B are respectively a plan view and a perspective view illustrating the configuration of a corner cube array. The corner cube array shown in FIGS. 3A and 3B is a cubic corner cube array in which a number of corner cubes, each being defined by three square planes that are opposed perpendicularly to each other, are arranged two-dimensionally. The shortest distance Pcc between the tops of two adjacent corner cubes will be referred to herein as an “arrangement pitch of corner cubes”.
Also, a method of making a corner cube array in which corner cubes of a very small size are arranged (and which will be referred to herein as a “micro corner cube array (MCCA)”) by etching anisotropically the surface of a substrate with a crystal structure was proposed by the applicant of the present application in Japanese Patent Application Laid-Open Publication No. 2003-066211. The arrangement pitch Pcc of the MCCA may be equal to or greater than the wavelength of visible radiation and equal to or smaller than the maximum pixel width of a reflective liquid crystal display device, for example.
If an MCCA is used as the interlevel dielectric layer 20 in the display device 100 shown in FIG. 2, then the following problems will arise.
In this display device 100, each pixel electrode 8 is electrically connected to the drain electrode 15 of its associated switching element 14 by way of a contact portion 16 that has been formed through the interlevel dielectric layer 20. The contact portion 16 has been formed in a contact hole that has been cut through the interlevel dielectric layer 20. If a contact hole is cut, however, the surface shape of the interlevel dielectric layer 20 partially collapses, thus decreasing the retroreflectivity of the reflective layer 2 to be deposited on the interlevel dielectric layer 20.
This problem will be described more fully with reference to the accompanying drawings.
FIGS. 4A and 4B are schematic cross-sectional views illustrating an ordinary process for forming the reflective layer 2. First, as shown in FIG. 4A, a contact hole 17 is cut through the interlevel dielectric layer 20 with the MCCA shape so as to reach the upper surface of the drain electrode 15 of a switching element 14. Thereafter, as shown in FIG. 4B, a conductive film is deposited on the inner surface of the contact hole 17 and on the interlevel dielectric layer 20 and then patterned, thereby forming the reflective layer 2 and the contact portion 16 out of the same conductive film. As can be seen from FIG. 4B, the predetermined surface shape is not formed in a portion 22 of the reflective layer 2 that is located over the contact portion 16, and therefore, the retroreflectivity decreases significantly in that portion 22 than the other portions of the reflective layer 2. That portion 22 of the reflective layer 2 will be referred to herein as a “non-retroreflective portion”. When there is such a non-retroreflective portion 22, the retroreflectivity of the reflective layer 2 decreases, thus causing a decrease in contrast ratio on the screen.
Also, the present inventors discovered and confirmed via experiments that when the reflective layer 2 was formed by the process shown in FIGS. 4A and 4B, the areas of the non-retroreflective portions 22 of respective pixels were too varied to realize uniform display performance. Hereinafter, this problem will be further discussed with reference to FIGS. 5A through 5D.
FIG. 5A is a cross-sectional view of contact holes 17a, 17b and 17c that have been made around the lowest-level point of a corner cube in the interlevel dielectric layer 20, around the highest-level point of another corner cube, and halfway between the lowest- and highest-level points of still another corner cube. As shown in FIG. 5A, the required depth of a contact hole changes according to the position of the contact hole in the corner cube. As used herein, the “depth of a contact hole” will refer to a distance from the surface of the interlevel dielectric layer 20 to that of a drain electrode exposed by the contact hole.
The respective depths of the contact holes will be described by way of specific examples. If the surface of the interlevel dielectric layer 20 has the shape of a cubic corner cube array with an arrangement pitch of 10 μm, then the height of each corner cube (i.e., the distance from a plane including a plurality of lowest-level points to the highest-level point) will be approximately 80% of the arrangement pitch, i.e., about 8 μm. Also, to protect switching elements (TFTs) with a thickness of 1 μm, the surface of those switching elements is preferably covered with an insulating layer that has a thickness of at least 2 μm. That is why the distance from the bottom of the interlevel dielectric layer 20 to the lowest-level points of the corner cubes may be 2 μm, for example. In such an example, the contact hole 17a made around the lowest-level point of a corner cube has a depth of 2 μm, the contact hole 17b made around the highest-level point of another corner cube has a depth of 10 μm, and the depths of the contact holes are variable within the range of 2 μm to 10 μm.
Generally speaking, to electrically connect two electrodes, which are located over and under the interlevel dielectric layer 20, respectively, with each other through a contact hole 17 of the interlevel dielectric layer 20 just as intended, it is important that the bottom size Sb of the contact hole 17 (i.e., the surface area of the drain electrode exposed through the contact hole 17) is at least equal to a predetermined value and that a cross section of the contact hole 17, as viewed perpendicularly to the surface of the substrate, has a predetermined taper angle (which will be simply referred to herein as a “taper angle”) Ta as shown in FIG. 5B. That is why every contact hole 17 of the interlevel dielectric layer 20 is preferably controlled so as to have substantially the same bottom size Sb and almost the same taper angle Ta.
However, if one attempts to equalize the bottom sizes Sb and taper angles Ta of the contact holes 17a, 17b and 17c with each other, then the sizes Su of those contact hole will change with the depths of the contact holes. That is to say, a contact hole 17a with a relatively small size Su is made around the lowest-level point of a corner cube, while a contact hole 17b, of which the size Su is larger than that of the contact hole 17a, is made around the highest-level point of another corner cube as shown in FIG. 5C. As used herein, the “size Su of a contact hole” means the size of a cross section of the contact hole at the uppermost portion thereof as viewed parallel to the surface of the substrate.
If a reflective layer 2 is formed on such an interlevel dielectric layer 20 having contact holes 17 of multiple different sizes Su, then the non-retroreflective regions 22 of the reflective layer 2 will have their areas varied with the sizes Su of the contact holes 17 as shown in FIG. 5D. For example, in a portion of the reflective layer 2 that is located over the contact hole 17a, the retroreflector shape of one corner cube is affected by the non-retroreflective region 22a. On the other hand, in another portion of the reflective layer 2 that is located over the contact hole 17b, the retroreflector shape of two corner cubes is affected by the non-retroreflective region 22b. As a result, uniform retroreflectivity is no longer achieved within the same display area and the brightness will vary on the same screen. Furthermore, the brightness also changes with the color to be presented (i.e., R, G or B), thus causing a coloring phenomenon in a white display mode, too.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2003-195788 discloses an arrangement that is designed to position all of those contact holes 17 around the lowest-level points of corner cubes by matching the arrangement pattern of switching elements (TFTs) 14 on the TFT substrate 16 to that of the corner cubes on the interlevel dielectric layer 20. According to this arrangement, the depths of the contact holes 17 can be substantially equalized with each other in all pixels, thus realizing a uniform display characteristic all over the display area. In addition, since all of the contact holes 17 are positioned around the lowest-level points of the corner cubes, the non-retroreflective regions 22 of the reflective layer 2 can have reduced areas. As a result, the deterioration of the retroreflectivity due to the variation in the area of the non-retroreflective regions 22 can be minimized.
To make a reflective display device such as that disclosed in Japanese Patent Application Laid-Open Publication No. 2003-195788, however, it is difficult to match the arrangement pattern of the corner cubes to that of the switching elements with high precision particularly when a corner cube array with a small arrangement pitch is used. Japanese Patent Application Laid-Open Publication No. 2003-195788 also proposes a method of forming the interlevel dielectric layer 20 using a transfer mold. According to such a method, it is necessary to make a different transfer mold for every type of TFT substrate for use in a display device, thus increasing the manufacturing cost unintentionally.
To overcome such a problem, the applicant of the present application proposed an arrangement, which is specially designed for a reflective display device to minimize the deterioration in the retroreflectivity caused by the non-retroreflective regions, in Japanese Patent Application Laid-Open Publication No. 2003-255373. According to this arrangement, the reflective layer can also maintain its predetermined MCCA shape even over the contact portions, thus reducing the overall area of the non-retroreflective regions formed by the contact portions as will be described in detail later.
FIG. 6 is a schematic cross-sectional view illustrating the arrangement of a reflective display device as proposed in Japanese Patent Application Laid-Open Publication No. 2003-255373. If any of the components of the reflective display device shown in FIG. 6 has the same function as the counterpart of the reflective display device shown in FIG. 2, that pair of components will be identified by the same reference numeral and the description thereof will be omitted herein for the sake of simplicity.
On the rear substrate 12 of the display device 200, stacked in this order are an interlevel dielectric layer 40, of which the surface has an MCCA shape, and a reflective layer 46 having the same surface shape as the interlevel dielectric layer 40. The reflective layer 46 is made up of a plurality of pixel electrodes 48. The interlevel dielectric layer 40 includes contact portions 44 to electrically connect each of those pixel electrodes 48 to its associated switching element 15. The surface of each contact portion 44 forms a part of the MCCA shape of the interlevel dielectric layer 40. That is why the reflective layer 46 can maintain its predetermined retroreflective shape even over the contact portions 44.
Japanese Patent Application Laid-Open Publication No. 2003-255373 also discloses a method of forming such an interlevel dielectric layer 40 and such a reflective layer 46 by a transfer process. Hereinafter, it will be described with reference to FIGS. 7A through 7D how to form the interlevel dielectric layer 40 and the reflective layer 46 according to the method disclosed in Japanese Patent Application Laid-Open Publication No. 2003-255373.
First, as shown in FIG. 7A, a layer to be patterned 30 is formed on a substrate 35 that already has a plurality of switching elements (TFTs)(not shown) thereon. The layer to be patterned 30 includes a resin layer 32 of an insulating resin and a plurality of conductive portions 34, which are arranged in the resin layer 32 so as to make an electrical contact with the drain electrodes of their associated switching elements. The resin layer 32 may be formed by applying a photosensitive acrylic resin, for example. The conductive portions 34 may be formed by cutting openings to reach the surface of the drain electrodes through the resin layer 32 and then filling the openings with a resin material with conductivity and photosensitivity (e.g., a conductive resin such as an acrylic resin), for example.
Thereafter, as shown in FIG. 7B, the uneven surface shape of a master (such as a die) 36 is transferred onto the layer to be patterned 30 by a stamping process, for example. The uneven surface shape of the master 36 may define the cubic corner cube array shown in FIG. 3, for example. The transfer process may be done by putting the master 36 at a predetermined position on the surface of the layer to be patterned 30 and then irradiating the layer to be patterned 30 with an ultraviolet ray with pressure applied on the master 36 toward the substrate 35.
When the master 36 is removed from the substrate 35 after that, an interlevel dielectric layer 40 such as that shown in FIG. 7C can be obtained. The interlevel dielectric layer 40 includes contact portions 44, corresponding to the conductive portions 34 of the layer to be patterned 30, and an insulating layer 42 corresponding to the resin layer 32 of the layer to be patterned 30. The surface of the contact portions 44 and the surface of the insulating layer 42 have the inverted shape of the uneven surface shape of the master 36 (i.e., has an MCCA shape).
Subsequently, as shown in FIG. 7D, a reflective layer 46 of a metal such as Ag is deposited on the surface of the interlevel dielectric layer 40 by a sputtering process or an evaporation process, for example, and then patterned if necessary.
According to this method, the interlevel dielectric layer 40 is formed by deforming not only the resin layer 32 but also the conductive portions 34 on the substrate 35 by a transfer process. Thus, the reflective layer 46 that has been deposited on the interlevel dielectric layer 40 also has the predetermined MCCA shape even over the contact portions 44 and has no regions at all with locally decreased retroreflectivity (such as the non-retroreflective regions 22 shown in FIG. 4B) as shown in FIG. 7D. Consequently, the reflective layer 46 can exhibit uniform and good retroreflectivity all over the display area while minimizing the deterioration or variation in retroreflectivity that would be caused by those non-retroreflective regions.
However, the present inventors discovered that the following problems happened when a reflective display device was fabricated by the method that has just been described with reference to FIGS. 7A through 7D.
According to the method described above, the two types of resins included in the resin layer 32 and conductive portions 34 of the layer to be patterned 30 cure and shrink independently of each other during the transfer process. In this case, if the resin materials of the resin layer 32 and conductive portions 34 had significantly different properties, then the surface shape might collapse, and the desired retroreflectivity could not be realized, in the boundaries between the resin layer 32 and the conductive portions 34 (i.e., in the boundaries between the insulating layer 42 and the contact portions 44). To avoid such a collapse, the range of selectable materials would be limited.
In the transfer process step of the process shown in FIGS. 7A through 7D, pressure (which will be referred to herein as “pressing pressure”) is applied on the master 36 and toward the substrate 35 after the master 36 has been put on the layer to be patterned 30 as described above. By applying this pressing pressure, not only the resin layer 32 but also the conductive portions 34 need to be deformed by the transfer process. However, the conductive portions 34 made of a conductor such as a metal have higher rigidity, and are deformed less easily with pressure or heat, than the resin layer 32. That is why to deform both the conductive portions 34 and the resin layer 32 with a pressing pressure at the same time, the pressing pressure needs to be increased compared to the situation where only the resin layer 32 should be deformed. If the pressing pressure is increased in the transfer process step, however, the rear substrate might suffer some damage or any other inconvenience might arise. As used herein, the “pressing pressure” refers to pressure applied per substrate on the transfer mold and toward the substrate.
This problem will be described more fully with reference to the accompanying drawings. FIGS. 8A and 8B are schematic cross-sectional views illustrating the transfer process step shown in FIG. 7B in further detail.
As shown in FIG. 8A, even if high pressing pressure is applied on the master 36 and toward the substrate 35, the yield point is reached earlier in the resin layer 32 of the layer to be patterned 30 than in the conductive portions 34. That is why most of the pressing pressure is used to plastically deform the resin layer 32 and does not reach the substrate 35. Meanwhile, in the conductive portions 34 of the layer to be patterned 30, the pressure on the conductive portion 34 is conveyed to the substrate 35 as pointed by the arrow 39 before the yield point is reached. As a result, some portions of the substrate 35 will be subjected to excessively high pressure locally and may suffer some damage as shown in FIG. 8B. For example, if a TFT substrate, including TFTs on a glass substrate, is used as the substrate 35, then the glass substrate may be partially broken under the pressing pressure applied in the transfer process as identified by the reference numeral 41.
Also, if the pressing pressure is high, then the highest-level points of the uneven surface of the master 36 might contact with TFTs or thin-film lines on the substrate 35. For example, if one of the highest-level points of the master 36 contacted with the channel region of a TFT, then the channel region will be exposed on the interlevel dielectric layer 20 formed by the transfer process and leakage current will flow between the source and drain electrodes of the TFT or any other inconvenience may be caused.
The conductive portions 34 may be made of an electrically insulating resin in which fine conductive particles (such as nanoparticles of a metal) are dispersed. In that case, if the number of those conductive particles is increased to maintain the electrical conductivity of the conductive portions 34, then that resin with conductive particles will have decreased elasticity and increased brittleness. When stamped, a highly brittle material might be broken and produce dust without being deformed with the master. And if such dust (i.e., electrically conductive foreign matter) has scattered in the insulating layer 42 of the interlevel dielectric layer 40, then electrical connection may no longer be maintained between a drain electrode and the reflective layer 46, leakage current may flow between adjacent drain electrodes or any other problem may arise. That is why it is difficult to increase the conductivity of the contact portions 44 without deteriorating the electrical insulating property of the insulating layer 42.
As described above, according to the method that has been described with reference to FIGS. 7A through 7D, not only the resin layer 32 but also the conductive portions 34 should be deformed in the layer to be patterned 30 by the transfer process. As a result, the desired retroreflectivity may not be realized or various problems may arise in the manufacturing process.
In order to overcome the problems described above, a primary object of the present invention is to improve the display performance of a retroreflective display device by increasing the retroreflectivity of its reflective layer.
Another object of the present invention is to provide a highly productive method for fabricating such a reflective display device easily.